Letter Circular 991: Motor Gasoline

, FLHsAM U, So DEPARTMENT OF COMMERCE 3.5 RATIONAL BUREAU OF STANDARDS 0 Washington 25 , D» C Letter Circular July 10, 1950 LC-99X Supersedes LC-551 and LG-281 MOTOR GASOLINE This letter circular is intended, to furnish basic information about gasoline that will assist the average motorist in selecting the proper fuel for his a,ut ©mobile Gasoline is composed essentially of a class of compounds known as hydrocarbons, which, as the name implies, are made up of carbon and hydrogen. These compounds differ not only in volatility, or the ease of evaporation, but also in other properties, notably in their tendency to cause combustion knock. The properties of the gasoline are therefore dependent on which of the various hydrocarbons are present, and in what proportions. Since crude petroleums vary considerably in composition, it may be seen that gasoline derived from these crudes may also be quite different. In addition, modem refining processes change the structures of some of the hydrocarbons to more desirable types, so that the products are still more complex,, There are certain minimum requirements that should be - and generally are - met by all gasolines. Gasoline should not contain more than a very small amount of gum, and if gum-forming constituents are present, the gasoline should also contain an inhibitor which is effective in preventing the formation of objectionable amounts of gum. The amount of materials tending to corrode or otherwise damage the engine should be below a satisfactory limit* The antiknock quality should be adequate for the engines in which it is to be used. The volatility range of the fuel should be suited to the season with respect to ease of starting, engine warmup, and vapor locko The principal difference between gasolines currently marketed is in the octane number which is a measure of the resistance of a fuel to combustion knock 0 The octane number of a fuel has no relation to its starting and warmup characteristics, or to its tendency to cause vapor lock. Many properties of gasoline are subject to some degree of refinery control. The more important of these are discussed in the following paragraphs. 1, Antiknock Quality of Gasoline The resistance of a gasoline to combustion knock is usually expressed as an octane number, a high octane number indicating a good resistance to combustion - 2 - knock and vice versa. The octane number is the percentage of isooctane in a mixture of isooctane and norma,! heptane which has the sane resistance to combustion knock as that of the gasoline when tested under specified conditions. When a gasoline has an octane number of say 75 its antiknock quality is equal to that of a mixture of 75 percent isooctane and 25 percent normal heptane. Octane numbers are determined on special test engines where the speed, operating temperatures, and other important factors can be precisely controlled, Fuels for automobile engines are rated according to two methods standard- ized by the American Society for Testing Materials, One of these (D-357 ) is known as the Motor Method, the other (D-908) as the Research Method, The Research Method test conditions are less severe and give higher ratings than the Motor Method, For example, two fuels having Motor Method ratings of 75 octane number ma,y have Research Method ratings of 77 and 81 octane number respectively. The spread between the two ratings measures the sensitivity of the fuel to the severity of test conditions. Thus the first fuel is comparatively insensitive, while the second is a typical sensitive fuel. The relative severity of automobile engine operating conditions determines whether the higher Research octane number of the more sensitive fuels will give improved antikock performance in any particular car. The average knock rating of regular grade gasoline, as marketed in different sections of the country, ranges at present from 75 to 80 Motor octane number and from 80 to 85 Research octane number. Some of the brands of regular gasoline available in a given area may differ in rating by several octane units 0 However, the fact that one gasoline has a somewhat lower Motor or Research octane number than another does not necessarily mean that it will have a greater tendency to knock in a given car. If the car knocks on a regular gasoline one should make sure that the ignition timing is not unduly advanced and then try other brands of regular gasoline before assuming that premium grade fuel is required. The average knock rating of premium grade gasoline ranges from 80 to 83 Motor octane number and from 87 to 91 Research octane number 0 The most important of the engine design factors influencing the tendency of gasoline to knock during combustion is the compression ratio of the engine (ratio of the volume filled by the charge at end of the intake stroke to the volume at end of compression). Increasing the compression ratio improves engine efficiency but also subjects the charge to higher temperatures and pressures, requiring fuel of higher octance number for knockfree performance. The average Motor Method rating of regular gasoline increased from below 63 octane number in 1930 to about 75 octane number in 1941 and the average rating of premium grade gasoline increased from 70 to 80 octane number in the same period. This has permitted the average compression ration of automobile engines to be increased from 5,15sl to 7,00sl since 1930, During the past twenty years, the specific output (horsepower per cubic Inch) of the average automobile engine has increased nearly 40 percent. - 3 - About three quarters of the cars now in service will give adequate antiknock performance on regular grade gasoline. Some of these cars may knock lightly at low speeds while accelerating or hill climbing, but the knock usually occurs only during a very small part of the total operating time and is not severe enough to affect engine power. In those cars which, due to engine design or condition, knock severely on regular gasoline, the knock can be moderated or eliminated either by retarding the spark or by using premium grade gasoline. Since knock and a retarded spark both tend to reduce engine power, the use of premium gasoline may be advantageous in this cane. However, premium grade gasoline will not improve the power and economy of any car which operates without objectionable knock on regular grade gasoline at the normal spark setting, 2® Volatility of Gasoline Under operating conditions air and gasoline spray enter the manifold of an automobile engine. Here they are intimately mixed by turbulence of the air stream, heated to some extent by contact with the walls of the manifold and enter the cylinders. Entrance through the intake valve still further increases the turbulence and consequent mixing of the charge. Generally, the liquid is not all evaporated at this stage; probably some of it is in the form of small droplets which are further vaporized by mixing with the residual hot exhaust gases in the cylinder and by heat generated during compression of the charge before ignition. Hence under ordinary running conditions little of the liquid should remain at the time of ignition. It can therefore be seen that the volatility of the gasoline is an important factor in satisfactory engine performance. Enough of the lower-boiling components of a fuel must vaporize in the cylinder to form an explosive mixture before a cold engine can be started. However, if there are too many low-boiling components in a gasoline, the fuel may boil and form vapor in the fuel lines, pump, or carburetor® When this occurs the engine may begin to misfire or may stop altogether. This failure 8 of the fuel system due to accumulation of vapor is known as "vapor lock' ® When an engine vapor locks it is usually necessary to wait for the engine to cool before it can again be started® Volatility also affects the ability of the engine to warm up properly. The volatility of a gasoline is measured in two ways® Eirst, a measure- ment of its vapor pressure will give an indication of its vapor-locking tendency. Second, a more complete analysis of its volatility may be made from its distillation curve. The table below shows the average distillation date, on regular and premium gasolines as reported in the Bureau of Mines surveys me.de in the summer of 1949, and winter of 1949-1950, - 4 - Table I Boiling Ranges of Average Gasolines Temperatures. °P Percent Summer 1949 Winter 1949-1.950 Evaporated Regular Premium Regular Premium IBP 100 101 90 91 5 120 120 105 105 10 137 135 122 120 20 165 159 150 144 30 191 182 177 168 50 237 227 226 216 70 283 273 272 265 90 345 338 336 331 95 371 365 363 358 End Point 404 397 398 394 The data in table 1 are averages for the entire United States; departures from these averages may he large in those areas where climatic conditions require more or less volatile fuel® It will he noted that the average temperatures for summer gasolines are somewhat higher than those for winter gasolines, and that the differences between regular and. premium gasolines are quite small, the largest difference being at or near the 50 percent point. (a) Effect of Volatility on Starting: The starting ability of a gasoline depends on the percentage of its components vaporizing at atmospheric temp- erature. It has been found that gasoline-air mixtures containing less than about 5 percent by weight of vaporized gasoline are not fired by a spark plug. 5 - A carburetor is designed to furnish air and fuel in varying proportions according to the conditions under which the engine is operating.

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